This invention relates to a solder composition and more particularly to solder compositions suitable for bonding a semiconductor die to plated and unplated metal package members.
In the fabrication of semiconductor devices including transistors, diodes, integrated circuits, and the like it is conventional to fabricate a semiconductor die and then to attach that die to a metal package member or header. The metal package member ultimately serves for mounting support, protection, electrical connection, and dissipation of heat generated in the operation of the device. The attachment is accomplished by using an epoxy resin or a metal alloy solder. Epoxy resin is generally unsatisfactory in most applications because of its inherently high electrical resistance and low thermal conductivity. The metal and metal alloy solders used for die attachment are conventionally divided into hard solders and soft solders. The hard solders, typically gold alloys, form strong bonds which have excellent thermal and electrical properties; but the high strength of the hard solder can cause the semiconductor die to fracture or the die and metal member to deform elastically under thermal stress because the hard solder transfers the stress to the die without plastic deformation in the die bond. The conventional soft solders, typically lead or tin alloys, are plastic enough to accommodate the thermal expansion mismatch between the die and the package, but are susceptible to metal fatigue after repeated temperature cycles. Such metal fatigue can result in device reliability problems. Thicker layers of the soft solder can reduce the stress in the solder and lessen the fatigue problem but only at the expense of undesirably higher thermal resistance. Intermediate solders such as the silver-tin-antimony alloys disclosed in application Ser. No. 788,954 assigned to the assignee of the present application and filed Apr. 19, 1977 now U.S. Pat. No. 4,170,472, have characteristics intermediate between the hard and soft solders and are able to overcome many of the problems attendant with the prior art solders. But all of these solders, hard, soft, or intermediate, require special surfaces to which to bond. This usually entails, for example, the plating of the package member with a layer or layers of gold, silver, nickel, or the like. Similarly, the back of the die itself is covered, usually by evaporation, with a layer of a suitable metal such as gold or silver. These special metals on the package and on the die are required because the solders employed either will not wet or will not adequately wet, the conventional package materials under standard bonding conditions. This requirement of specially plated packages greatly adds to the expense of the package both because of additional processing steps required and because of the expense of the plating metal itself.
Accordingly, a need existed to provide solder alloy compositions which would overcome the problems of prior art compositions to produce a high yielding, low cost composition and method for the attachment of semiconductor die to metal package members.
It is therefore an object of this invention to provide an improved solder alloy composition for the attachment of semiconductor die to plated or unplated metal package members.
It is a still further object of this invention to provide a solder composition for semiconductor die attachment having desirable electrical, thermal, and thermal fatigue resistant properties.